A much-postponed launch of a spacecraft meant to explore two of the solar system's largest asteroids has been delayed again, this time until September.

NASA didn't want the launch of the Dawn spacecraft, which had already been delayed until July 15, to conflict with preparations for the August launches of the space shuttle Endeavour and the Phoenix Mars lander, said NASA spokesman George Diller.

When the spacecraft does launch, it will embark on a years-long journey to the asteroids Vesta and Ceres, between Mars and Jupiter.

The spacecraft will first visit Vesta, the smaller of the two bodies, four years from now. In 2015, it will meet up with Ceres, which carries the status of both asteroid and, like Pluto, dwarf planet.

NASA has rescheduled the launch several times. The original launch, planned for Saturday, was canceled because storms at the launch pad prevented loading its fuel. Also, a plane used to track the spacecraft after liftoff was having mechanical problems, and the tracking ship wasn't in the correct location.

The launch date was changed to Monday, and then July 15, before being postponed Saturday until September.

NASA has until the end of October to launch the spacecraft before the planetary bodies begin to drift apart. It will take another 15 years for them to get back together again, said Chris Russell, the mission's principal investigator.

The $344 million mission has had more than its share of reversals: Last year, the $344 million mission was killed because of cost overruns and technical problems. The Jet Propulsion Laboratory in Pasadena, which manages the spacecraft, appealed to NASA Administrator Michael Griffin and got the project revived.

Adding to the drama, Ceres briefly flirted with planethood during last summer's scientific debate about whether Pluto is a planet. Both Pluto and Ceres were finally classified dwarf planets.